The hydrolysis rate of chlorpyrifos,O-O-diethylO-(3,5,6-trichloro-2-pyridyl) phosphorothioate, and its dimethyl analog, chlorpyrifos-methyl, in dilute aqueous solution

Abstract

The hydrolysis rate of chlorpyrifos (the active ingredient of DURSBAN® and LORSBAN® insecticides, registered trademarks of The Dow Chemical Company, Midland, Michigan) in water followed simple first-order kinetics over the concentration range, 3×10⁻⁹ to 3×10⁻⁷ M. In buffered distilled water at 25°C and pH 8.1, 6.9, and 4.7, the half-life was 22.8, 35.3, and 62.7 days, respectively. Additional data were obtained for rates at 15° and 35°C. The activation energy for the reaction under these conditions was 21.2 kcal/mole. A 16-fold rate enhancement was demonstrated in canal and pond water at 25°C. Likewise, there was a catalytic effect on the hydrolysis rate in the presence of copper (II) ion.

The same information was generated for chlorpyrifos-methyl, the dimethyl analog of chlorpyrifos. In this case, the corresponding half-life values for hydrolysis at 25°C and pH 7.8, 6.7, and 4.2 were 12.7, 17.4, and 22.8 days, respectively. The activation energy was 20.8 kcal/mole, not significantly different from that for chlorpyrifos. A hydrolysis rate enhancement also occurred for chlorpyrifos-methyl in canal water.

Qualitatively, the products of chlorpyrifos hydrolysis were 3,5,6-trichloro-2-pyridinol (I),O-ethylO-hydrogenO-(3,5,6-trichloro-2-pyridyl) phosphorothioate (II), andO, O-dihydrogenO-(3,5,6-trichloro-2-pyridyl) phosphorothioate (III). In the case of chlorpyrifos-methyl, the hydrolysis products were compound I and the methyl analog of compound II.